Hepatitis A virus has been successfully adapted to growth in African green monkey kidney tissue culture. Over 20 serial passages have been achieved, and infectivity titers as high as 10 to the eight infectious units per ml of cell concentrate have been measured. The virus is predominantly cell-associated and does not produce CPE. It was attenuated for chimpanzees after 10 serial tissue culture passages; reevaluation of the virus in chimpanzees after 20 tissue culture passages is in progress.